1. Field of Invention
This invention relates generally to the field of reflector-type antennas and, more specifically, to reflector-type antennas based on the offset Gregorian configuration.
2. Description of Prior Art
Financial support from the SETI Institute, made possible by the Paul G. Allen Foundation, is gratefully acknowledged.
The present invention relates to reflector-type antennas and antenna systems based on the offset Gregorian design. Reflector antennas are described in several references including, for example, “Reflector Antennas” by K. S. Kelleher and G. Hyde appearing as Chapter 17 of the “Antenna Engineering Handbook,” 3rd Ed., Richard C. Johnson Ed. (McGraw-Hill, 1993), the entire contents of which is incorporated herein by reference for all purposes.
The Gregorian design was first used as an optical reflecting telescope and named after its inventor, James Gregory, who described it in 1663. The optical Gregorian telescope comprises a parabolic primary reflector serving as the objective, and a concave, elliptical secondary reflector located on the optical axis beyond the primary focal point. The image is formed behind the primary parabolic reflector through which a hole has been bored.
The basic Gregorian structure as used in optical telescopes may be adapted for use as an antenna operating with wavelengths longer than the optical region of the spectrum, typically as a radio telescope operating in the region of microwave or radio frequencies. Typically, a configuration known as an offset Gregorian design may be implemented wherein the secondary reflector is positioned off the primary axis. This structure has many benefits such as improved beam efficiency, greater effective area, and lower sidelobe levels. However, feed spillover onto the ground from this design may carry a potential for increased background noise, leading to lowered sensitivity and increased signal collection times. This is a particularly important characteristic for applications in radio astronomy in which the goal is typically to detect, collect, and analyze faint signals emanating from the sky.
The reciprocity theorem for antennas is a well-known and often-used theorem showing that the performance of an antenna is the same whether it is used in reception or transmission, provided however, that no non-reciprocal devices (such as diodes) are present. For the typical cases considered herein, the reciprocity theorem applies and we describe the performance of antennas either in transmission or reception without distinction. That is, when used for transmission, electromagnetic energy is delivered to the antenna for transmission by means of a “feed.” When used in reception, energy collected by the antenna is delivered to a “detector” for detection and delivery to various electronic or other signal processing means. In the descriptions herein, the reciprocity theorem is employed and feeds or detectors are described as components of the antenna or antenna system without distinction, unless specifically noted.
Therefore, in light of the above description, a need exists in the art for systems and methods to maintain the benefits of the offset Gregorian antenna design while reducing background noise and ground scatter. Addressing this need would result in an antenna with improved performance and, for a particular application example, a radio telescope with improved sensitivity and improved signal collection efficiency.